Electrical joints in an electrical circuit are internationally recognized as one of the major causes of electrical failures in the electrical circuit. At the electrical loads associated with commercial use, failures of such electrical joints often results in an arc flash.
The arc flash causes an explosion which may result in a serious if not fatal injury to any person in close proximity to the compromised electrical joint when it fails. Furthermore, severe damage will be caused to the electrical equipment itself and there will be a loss of power to the affected circuits.
Thus the failure of an electrical joint can have serious economic, environmental and safety consequences. This is particularly applicable in organisations which have high downtime costs such as data centres, oil and gas production, refining and other high value large scale manufacturing sites.
The issue of electrical joint failure is so serious that commercial insurers often mandate the client to undertake annual inspections of their electrical equipment in general and particularly any mission critical electrical equipment which may result in the cessation of the business or for business to be severely interrupted should it fail.
The electrical joints may be, for example, joints between two sections of metal conductors, which are typically formed from copper or aluminium, or at the termination of a section of an electrical cable to a metal conductor.
At present the only way to detect if the integrity of an electrical joint has been compromised is to detect excess heat at the electrical joint. There are currently two methods used to detect such compromised joints.
The first method is periodic inspection of the electrical joints using a thermal imaging camera. Such inspections will be carried out whilst the electrical circuit and thus the electrical joint is energised with typically 50% or less of electrical load designed for the electrical circuit and thus the electrical joint. Depending on the particular country and the regulations in force the energised joints may or may not be exposed during the inspection. When the joints are not exposed the inspection is carried out externally, sometimes using a thermal window to enhance the level of infrared radiation visible to the thermal imaging camera.
The problem is that such inspections are often only carried out annually, i.e. 1 day out of 365, less than 1% of the operating time of the electrical circuit. As such any problems which develop with the electrical joints, to cause them to become compromised, may go unnoticed for a long period of time. Furthermore if the inspection is carried out externally a calculated correlation of the external temperature to the internal temperature of the electrical joint will need to be carried out. Given the number of variables at each location, the correlation is subject to errors, which may be large. In addition as such inspections are carried out at unknown or low electrical loads, i.e. below 50%, the inspection is often unable to determine that an electrical joint is compromised.
The second method is a more recent technology which enables thermal measurement of the joint to be carried out using passive thermal sensing devices which are located inside the electrical enclosure in which the electrical joint is housed to directly and continuously monitor the electrical joint. The thermal sensing devices measure the ambient temperature of the air in the enclosure in which the electrical joint resides, and the temperature of the electrical joint to calculate the temperature differential of the electrical joint, known as the ΔT value.
The temperature of the electrical joint uses a sensor which is typically either an infrared sensor located a short distance from the electrical joint which either measures the temperature of the electrical joint itself or the temperature of the electrical conductor adjacent the electrical joint, or a cable sensor which is mounted directly onto cable conductors adjacent the electrical joint. Other sensing devices may be used provided that they measure the temperature at the electrical joint itself or of the conductor adjacent to the electrical joint.
There are problems with both of these methods in that whilst it is theoretically possible to subsequently identify the electrical load that was applied to the electrical circuit on which a given electrical joint is located, and then correlate this with the ΔT value from the thermal inspection to determine whether or not a joint is compromised, this is not always so simple in practice.
In an organisation with many electrical circuits, and thus electrical joints, this process would be very time consuming, and thus unlikely to be commercially viable.
Furthermore thermal inspection reports are not typically integrated with computer building management systems, thus data would need to be collected from a number of sources which may have different time stamps, therefore such calculations are not always reliable, nor can they dynamically predict how much additional load can be safely applied to an electrical circuit which is at a low electrical load, i.e. the maximum safe operating capacity, at any given time.
The determination of how much extra electrical load can be safely added to an electrical joint in an electrical circuit is particularly important in organisations which operate systems with dual electrical feeds. Quite often an electrical feed will be shut down for periodic maintenance. This results in both electrical feeds being fed into a single feed. Thus, where both feeds originally operated at 30% electrical load, a single feed is now operating at 60% electrical load.
Such sudden increases in electrical load often result in a compromised electrical joint failing, as whilst the compromised electrical joint could cope with 30% electrical load, the compromised electrical joint could not cope with 60% electrical load. Neither periodic thermal inspection nor continuous monitoring would have been able to detect that such an electrical joint was compromised at low electrical load levels.
A further problem is that electrical circuits, and thus electrical joints normally have designated maximum operating temperatures imposed by the manufacturers, i.e. the maximum temperature at which they can be safely operated. Furthermore, there may be other maximum operating temperatures imposed by industry standard organisations such as UL, IEC and ANSI. However, this temperature is affected by both the electrical load being applied to the electrical joint and the local ambient temperature.
Thus if the local ambient temperature is high it may be necessary to reduce the electrical load passing through the electrical joint to ensure that the maximum safe operating temperature is not exceeded.
For example;
The maximum temperature at which an electrical joint can be operated at 100% load is dependent on the particular standards being adhered, i.e. British Standards, UL, IEC or ANSI standards. These standards stipulate a maximum rise of 50° C. above a 24 hour mean ambient temperature of up to 35° C. with an absolute maximum of 85° C., and peak ambient temperature of 40° C. with an absolute maximum of 90° C. ANSI alternatively permits a temperature rise of 65° C. above a maximum ambient temperature of 40° C. with an absolute maximum of 105° C. provided that silver plated terminations (or acceptable alternative) are provided in the electrical joint, if not then a maximum temperature rise of 30° C. is allowable with an absolute maximum of 70° C. For example, a manufacturer would have used these standards to calculate that the maximum ambient temperature that a particular electrical joint can operate at 100% electrical load is 50° C., as the ΔT value for the joint is 65° C. Thus if the ambient temperature rises to 60° C. the maximum load that the electrical joint could be used with safely is that which gives a ΔT of 55° C.
There is currently no system which enables effective monitoring of ambient temperatures and ΔT values of an electrical joint or an electrical conductor assuming the electrical joint is not compromised to indicate the maximum electrical load which can be passed though the given electrical joint or electrical conductor at a given ambient temperature.